The invention relates generally to the art of offshore working platforms and more particularly to an offshore platform designed for use in Arctic environments where the platform must resist impinging ice.
Arctic and subArctic offshore areas are currently the focus of extensive surveying for oil and mineral resources. Sea ice is the major challenge to such offshore exploratory and recovery operations. However, many areas are sufficiently shallow to permit the use of appropriately designed platforms which would gain stability by resting on the bottom. Shallow offshore operational areas suitable for the location of frozen drilling platforms extend from the maritime coastlines of western Alaska and Canada, east to Baffin Bay and Davis Strait. Shallow lagoons, typically several miles long by a mile or two wide, separate the mainland from paralleling island chains. Lagoon ice rarely forms mid-winter floes as is the case in the sea, and, after summer breakup, the ice is "rotten" and has very little destructive capability.
Offshore of the islands, the sea gradually deepens to perhaps as much as 100 feet at 25 miles distance from the shoreline and accordingly sea ice conditions are encountered. The destructive capability of this ice is too severe for conventional offshore platforms with straight, vertical legs. Conventional platforms supported in this manner are in use as far north as Cook Inlet in Alaska. The tide-driven Cook Inlet ice fails by crushing as it impinges against platform legs. Colder and thicker Arctic ice in sheet form or floe is too strong for this failure mode alone and it is generally more desirable to induce failure by bending to take advantage of the relatively weak tensile strength of ice. Ice breaking ships employ this principle. Icebergs will penetrate the area in which the instant invention will be used, but, like ice islands, the frequency of occurrence is extremely small. In addition to this, their extreme draft will cause them to ground before reaching the shallower drilling sites. Blasting smaller bergs into several parts by explosive charges would eliminate any hazard they might present.
Annual and polar sheet ice, ridged and rafted ice, bergy bits, and movement of ice frozen fast to the drilling platform are major considerations in design. Ridged and rafted ice, and bergy bits present a relatively tall advancing front. Despite their concentrated mass, inherent weaknesses are caused by ice in block form, cracks and internal stresses. The mass below water level is warmed and weakened by the sea water heat source.
Annual ice usually forms off the north slope in late September. During the early stages of its growth, it is relatively thin and susceptable to break up into floes during storms. The impact of floes against one another or shore-fast ice creates pressure ridges and rafts. Sheet ice in the floes continues to thicken during the remainder of the winter to mid-May. However, the floes are usually unstable and may develop open leads or new ridges. By June, ice growth has terminated and weakening occurs until break up around mid-July. Depending upon winds, this rotten ice may or may not be present until freeze-up. During the growing season, annual ice floes are active 12 miles or so offshore. These floes in March through early May are strong in compression and have great impact momentum. Closer in-shore similar strength floes have less impact momentum, but are more liable to fuse themselves to structures before moving.
In the mid to late winter period when annual and polar sheet ice has high compressive strength, it is weaker in tensile or bending strength. This weakness can be used by forcing the ice to bend as it rides up on sloping sides designed into a structure. The most severe situation develops if ice in large sheet form becomes fused to external surfaces. Several methods of preventing or reducing the strength of such bonds have been considered. The most promising approaches are to discharge air or condenser sea water under the ice. Either of these measures will weaken sea ice in a contact area allowing it to move and thus promote failure in tension.